1. Field of the Invention
The present invention relates to a multi-channel magnetic resonance imaging reconstruction method for water-fat separation, and more particularly to a Dixon method-based multi-channel magnetic resonance imaging reconstruction method for water-fat separation.
2. Description of the Prior Art
In magnetic resonance imaging (MRI), hydrogen protons in adipose tissues and hydrogen protons in other tissues of the human body have different resonance frequencies because they are in a different molecular environment. Also, after hydrogen protons in adipose tissues and other tissues are simultaneously excited by radio frequency pulses they have different relaxation times. Signals collected for adipose tissues and non-adipose tissues in various echo times have different signal strength.
The Dixon method is a method used in MRI for producing an image of pure water protons, the basic principle of which is: collecting, respectively, both in-phase and opposed-phase echo signals of protons in water and fat, and performing calculations on the signals of the two different phases to remove the fat signal, producing an image of pure water protons and thus achieving fat suppression. The Dixon method has disadvantages in that it is affected, to a relatively large extent, by non-uniformity of magnetic field, exposed to respiratory movements, and based on a complex calculation technique that is subject to errors.
An improved three-point Dixon method has been widely used so as to obtain images of water and fat at one time, and the principle thereof is: acquiring one in-phase image and two opposed-phase images and at the same time, calculating an additional phase caused by non-uniformity of the magnetic field from the two opposed-phase images, performing phase correction on the two opposed-phase images, and then using the two opposed-phase images together with the in-phase image to obtain images of water and fat.
The disadvantages of the three-point Dixon method lie in that the additional phase calculated from the two opposed-phase images can not be directly used to perform phase correction on the images, for the fact that the additional phase due to non-uniformity of magnetic field may exceed, that is to say, phase wrapping occurs and therefore there is a need for performing phase unwrapping. However, phase unwrapping is mathematically a problem without a solution, and in water-fat separation techniques, in order to perform phase unwrapping, it is assumed that the spatial change in non-uniformity of the magnetic field is slow, so the calculation process of phase unwrapping is quite time-consuming.
When multi-channel coils are used for imaging, in order to obtain merged images of water and fat, images of water and fat of respective channels are calculated respectively, that is to say, for each of the channels, the two opposed-phase images calculated are used to calculate the additional phase caused by the non-uniform magnetic field to perform phase unwrapping, the unwrapped phase is used to perform phase correction on the two opposed-phase images, and finally, images of water and fat are obtained; and then the images of water of respective channels and the images of fat of respective channels are merged, respectively, to obtain the final result.
The above-described scheme has several problems, as follows:
(1) reconstruction time is too long;
(2) for channels which have a poor image signal-to-noise ratio (SNR), images of water and fat obtained are unreliable; and
(3) due to inherent instability of phase unwrapping, for some channels, the images of water and fat calculated may be exchanged, causing errors in the merged images of water and fat.
Due to the inherent disadvantages of phase unwrapping, a new scheme has been proposed: collecting three images of 0, π/2, π finding, from the three images, direction vectors representative of fat and water signals and based on physical characteristics thereof, determining the correct solutions, then calculating a phase difference caused by the non-uniform field from the vectors of fat and water signals to correct the phase difference between the latter two images, and calculating images of water and fat.
However, this procedure also has several problems, as follows:
(1) since signals of respective channels must be processed respectively, reconstruction time is long; and
(2) for channels or regions which have a poor image SNR, the images of water and fat obtained are unreliable and the merged images of water and fat derived therefrom are unreliable as well.
Therefore, the inability to provide a Dixon method-based multi-channel reconstruction method for water-fat separation that eliminates the problem of reconstructing images of water and fat that are unstable and that take a long time is a problem in MRI technology in need of a solution.